NANO LETTERS
Preparation of Mesoscopic Gold Rings Using Particle Imprinted Templates
2004 Vol. 4, No. 7 1193-1196
Feng Yan† and Werner A. Goedel*,†,‡,§ Organic and Macromolecular Chemistry, OC III, UniVersity of Ulm, D-89069 Ulm, Germany, Inorganic Chemistry-Material and Catalysis, AC II, UniVersity of Ulm, D-89069 Ulm, Germany, and BASF Aktiengesellschaft, Polymer Research, Polymer Physics, 67056 Ludwigshafen, Germany Received February 20, 2004; Revised Manuscript Received May 18, 2004
ABSTRACT Polymer membranes with closely packed two-dimensionally arranged pores have been prepared at the water surface using monolayers of silica colloids as templates. The resulting porous membranes were used as templates for the preparation of gold rings via filling of pores with a solution of gold precursor followed by calcinations. The size of the resulting rings can be easily controlled by tuning the pore size of the templating membrane.
The properties of ring shaped materials change dramatically when their size shrinks to the meso- and nanoscopic regime. For example, (i) the trapping of a magnetic flux in the interior of an electrically conducting ring can lead to “persistent currents”.1-4 (ii) The optical properties of metal rings significantly deviate from the properties of compact spheres and can be tuned by varying the diameter and wall thickness of the ring.5 Therefore, methods to synthesize mesoscopic rings are important in the field of electronics and devices. Several methods for the preparation of rings made of semiconductors, metals, polymers, and other materials have been recently reported.6-12 For example, capped InAs quantum rings can be prepared by using molecular-beam epitaxy and annealing.6-8 Carbon rings have been produced by chemical modification of carbon nanotubes.9 Of particular interest are rings made from stable conducting metals that have potential use in the study of persistent currents and in microdevice applications.10 Gold rings have been prepared by using suitable structured templates, such as porous alumina,10a nanochannel glass,10b or spheres5 in combination with metal deposition and subsequent removal of unwanted metal via ion beam etching. However, these methods still need sophisticated equipment. More recently, nonconducting polymer tubes and ceramic rings have been simply prepared by selective wetting of porous membranes with polymer or metal alkyloxides. The following removal of porous templates yields the corresponding tubes and rings with uniform sizes.11,12 * Corresponding author. E-mail:
[email protected];
[email protected]. † Organic and Macromolecular Chemistry, OC III, University of Ulm. ‡ Inorganic Chemistry-Material and Catalysis, AC II. § BASF. 10.1021/nl0497169 CCC: $27.50 Published on Web 06/17/2004
© 2004 American Chemical Society
Here, we show that gold rings can as well be prepared in a simple manner by selective wetting of porous templates using polymer membranes with two-dimensionally arranged pores as moulds. Porous polymer membranes were derived from the polymerizable organic liquid trimethylolpropane trimethacrylate (TMPTMA) and silica colloids, as has been described in ref 13. Mixtures of the colloids and the organic liquid in a binary solvent (ethanol/chloroform (1:1 by volume)) were spread onto the water surface, in such an amount as to yield complete coverage of the water surface by the colloids. After the evaporation of the volatile solvent, the colloids formed a two-dimensional layer, which was embedded in the layer of the organic liquid. The organic liquid in the mixed monolayers was cross-linked by irradiation with UV light of 250 nm wavelength (low-pressure mercury arc lamp, Umex Co., Germany). Then the solidified composite films floating on the water surface were transferred onto mica sheets.To remove the silica colloids, the transferred films were exposed to the vapor of hydrofluoric acid (HF) for 4-5 min in a sealed plastic container. (CAUTION: Hydrofluoric acid is extremely corrosiVe and hazardous; it should be handled with care). Figure 1 shows the schematic procedure that is used to prepare gold rings. A porous membrane supported by a mica sheet was dipped into a solution of HAuCl4 (0.25 wt % in ethanol) for 2-3 min. After withdrawing it from the solution, the excess solution on the membrane surface was removed by touching the edge of the membrane with a filter paper. The filled samples were allowed to dry in air at room temperature for 30 min and then dried in vacuum. Calcination
Figure 1. Scheme of the preparation of gold rings on a mica sheet.
at 450°C in air simultaneously removed the porous polymer membrane and converted the HAuCl4 into metallic gold. Figure 2A shows the scanning electron microscopy (SEM) image of the templating porous polymer membrane prepared by using 330 nm silica colloids. The membrane has a thickness of approximately 190 nm, and a pore size of approximately 310 nm. All the pores are fully open on both the top and bottom surfaces of the membrane. Since the porous membranes are synthesized with the spherical particles as templates, the inner surface of the pores is concave. In the middle of the pore walls, one observes small circular “windows” on the walls between the pores. These openings presumably form in the position where two particles touched each other. Figure 2(B-D) shows the SEM images of patterned arrays of gold rings on a mica sheet prepared from the porous membrane as described above. In domains comprising several dozens to several hundreds of rings, one observes hexagonal order. This order is reminiscent of the
arrangement of pores in the porous membrane templates, and is not disrupted during the calcinations. The shape and size of the rings are relatively unchanged in comparison to those of the pores in the templates (Figure 2C, 2D). Since the inner surface of the pores is concave, the outer surface of the rings is convex correspondingly. Gold rings prepared from this template have an outer diameter, height, and wall thickness of approximately 300, 150, and 22 nm respectively. It is of interest to note that some of the rings have very small branches on the outer surface, which connect to neighboring rings (Figure 2C). The formation of these branches is probably due to the selective absorption of HAuCl4 solution on the inner surfaces of the circular “windows” between the pores. Most of these branches were disrupted due to some shrinkage of the gold during the calcination. The size of the rings can be easily controlled by preparing membranes with suitable pore sizes. Figure 3A shows gold rings prepared from porous polymer membranes with 110 nm pore size. The gold rings have an outer diameter and wall thickness of 105 and 25 nm respectively. Larger gold rings with outer diameters of 550 and 95 nm wall thickness can be prepared from porous polymer membranes with 560 nm size pores (Figure 3B). The concentration of HAuCl4 solution also affects the formation of the gold rings. If the concentration of HAuCl4 solution is increased, gold bowls (Figure 4A) or even particles (Figure 4B) can be formed on the substrates. The decomposition of HAuCl4 into metallic gold was confirmed by energy-dispersive X-ray spectroscopy and X-ray diffraction. Since the substrate is not completely covered by a continuous layer of gold, the EDX (Figure 5A) spectra show predominantly peaks due to the mica substrate (and an iron peak due to an artifact within the instrument). In addition to these peaks, one observes peaks corresponding
Figure 2. SEM images of (A) a porous membrane prepared by using 330 nm silica particles as templates, transferred onto a mica sheet (partially removed by a scotch tape to allow a side view of the pores). (B-D) SEM images of gold rings on mica, prepared using molds of porous membranes shown in (A). Images were obtained using a DSM 962 (Zeiss, Germany) and a S-5200 (Hitachi, Japan) SEM. 1194
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Figure 3. SEM images of gold rings of various sizes: (A) gold rings with diameter of ∼100 nm, (B) gold rings with diameter of ∼500 nm. Insets show the side view of the corresponding porous polymer membranes that were used as moulds.
to metallic gold. In the X-ray diffraction (Figure 5B) one as well observes predominantly peaks due to the mica substrate; the diffraction of the substrate decorated with gold rings shows, in addition, well visible peaks corresponding to the (111) and (222) Bragg reflections of gold. These peaks are in agreement with those reported for Au nanoparticles in the literature.14 From the width of the (111) Bragg reflection and the Debye-Scherrer equation,15 one can estimate that the gold crystallites have a size of approximately 24 nm. This size is in accordance with the wall width of the rings as it appears from the electron microscopy images. The method used in this work is based on the selective wetting of the inner surface of a porous polymer membrane by a gold precursor solution. However, the detailed mechanism of the formation of gold rings is not yet clearly understood. Our understanding of this process is as follows. After the dipping process, the pores are completely filled with the HAuCl4 ethanol solution. The liquid level in the pores was then decreased slowly due to the evaporation of the solvent. During this process, the increasingly concentrated solution selectively wets the inner walls of pores. This selective wetting might be due to a chemical contrast between the inner surface of pores and the outer planar surfaces of the porous membrane. In this work, the surface of the silica colloids was hydrophobized by coating with 3-(trimethoxysilyl) propyl methacrylate. The polymerizable methacroyl groups in the coating layer will link or copolymerize with Nano Lett., Vol. 4, No. 7, 2004
Figure 4. SEM images of structures obtained by dipping a mica substrate covered by a polymer membrane with 300 nm pores into solution of HAuCl4 in ethanol of concentration of 0.5 wt % (A) and 1 wt % (B).
the TMPTMA monomer during the polymerization. After the removal of the colloids, the inner surface of the pores has a different chemical composition than the outer surface of the membrane. Perhaps this chemical contrast causes the selective wetting of gold precursor onto the inner surface of pores. This can also explain why the outer planar surface of the porous membrane is almost free of a gold layer. However, this selective wetting might as well be simply due to the geometry of the pores. While the diluted solution initially completely fills the pores of the membrane, it retracts from the bottom to form a ring along the pore wall at some point before the auric acid finally precipitates and solidification occurs. The formation of the bowl-like structures indicates that at a higher concentration of the auric acid; solidification takes place earlier than the retraction from the bottom of the pore. The size of the porous membranes prepared by our method is limited only by the water surface. In our work, the porous membranes with pores of uniform size as large as 23 cm2 have been prepared.13 Thus, it is worth noting that using this comparatively simple method, areas exceeding the previously reported square millimeters can be covered with ordered arrays of gold rings. In conclusion, we have presented a simple and effective method for preparing gold rings by using porous membranes 1195
rings. By using this method, the size of the gold rings can be easily controlled by preparing the appropriate porous membrane templates. Acknowledgment. The support by M. Mo¨ller, B. Rieger, and K. Landfester (University of Ulm), H. Auweter and R. Iden (BASF) is greatly appreciated. We thank Prof. P. Walther and A. Ding for helping in high resolution SEM measurement. The authors also appreciate Dr. X. Zhang for X-ray measurement. This work was funded by the Deutsche Forschungsgemeinschaft (SFB 569, SPP 1052). References
Figure 5. Energy-dispersive X-ray spectra (A) and X-ray diffraction (B) of a mica sheet decorated with 300 nm diameter gold rings (thick continuous line) and of a bare mica sheet (thin continuous line). Peaks marked with “Au” indicate the presence of crystalline metallic gold. EDX spectra were recorded within the scanning electron microscope. X-ray diffraction was performed with a Philips powder diffractometer with a Cu-KR source (wavelength 1.54 Å).
as templates. These membranes have been demonstrated as good adsorption sites for the preparation of isolated gold
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(1) Matveev, K. A.; Larkin, A. I.; Glazman L. I. Phys. ReV. Lett. 2002, 89, 096802. (2) Rabaud, W.; Saminadayar, L.; Mailly, D.; Hasselbach, K.; Benoit, A.; Etienne, B. Phys. ReV. Lett. 2001, 86, 3124. (3) Bagci, V. M. K.; Gu¨lseren, O.; Yildirim, T.; Gedik, Z.; Ciraci, S. Phys. ReV. B 2002, 66, 045409. (4) Lorke, A.; Luyken, R. J.; Govorov, A. O.; Kotthaus, J. P.; Garcia, J. M.; Petroff, P. M. Phys. ReV. Lett. 2000, 84, 2223. (5) (a) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Ka¨ll, M.; Bryant, G. W.; Abajo, F. Phys. ReV. Lett. 2003, 90, 057401. (b) Winzer, M.; Kleiber, M.; Dix, N.; Wiesendanger, R. Appl. Phys. A 1996, 63, 617. (6) Garcia, J. M.; Medeiros-Ribeiro, G.; Schmidt, K.; Ngo, T.; Feng, J. L.; Lorke, A.; Kotthaus, J.; Petroff, P. M. Appl. Phys. Lett. 1997, 71, 2014. (7) Warburton, R. J.; Scha¨flein, C.; Haft, D.; Bickel, F.; Lorke, A.; Karrai, K.; Garcia, J. M.; Schoenfeld, W.; Petroff, P. M. Nature, 2000, 405, 926. (8) Raz, T.; Ritter, D.; Bahir, G. Appl. Phys. Lett. 2003, 82, 1706. (9) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Science 2001, 293, 1299. (10) (a) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.; Johnson, M. B. Nano Lett. 2004, 4, 167. (b) Pearson, D. H.; Tonucci, R. J.; Bussmann, K. M.; Bolden, E. A. AdV. Mater. 1999, 11, 769. (11) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Go¨sele, U. Science 2002, 296, 1997. (12) (a) Yi, D.; Kim, D. Nano Lett. 2003, 3, 207. (b) Xu, H.; Goedel, W. A. Angew. Chem., Int. Ed. 2003, 42, 4696. (13) Yan, F.; Goedel, W. A. Chem. Mater. 2004, 16, 1622. (14) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Ramani, R.; Parischa, R.; Ajayakumar, P. V.; Alam, M.; Sastry M.; Kumar, R. Angew. Chem. Int. Ed. 2001, 40, 3585. (15) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedure; John Wiley & Sons: New York, 1974; p 656.
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Nano Lett., Vol. 4, No. 7, 2004
